As use of radio frequency (RF) devices in telecommunications networks increases, a higher and higher number of devices are trying to communicate over a limited amount of RF spectrum. In next generation networks, it may be desirable to overcome this by managing the use of the RF spectrum through more flexible protocols and policies.
Typically, in a radio communication cell forming part of a wider telecommunications network, a finite amount of RF spectrum is divided into a plurality of discrete channels. Dynamic spectrum access (DSA) technologies allow access to those channels within a radio communication cell in a dynamic manner, reducing inefficiency in RF spectrum usage and so providing gains in terms of network capacity.
In some DSA technologies, primary UEs (PUs) are served by a base station in a radio communication cell and have license to use the channels of the RF spectrum provided by that base station as they require services such as data and voice. In general, all of the channels are not in use all the time, which provides the opportunity for use of those channels by unlicensed users. In this sense, secondary UEs (SUs) may be defined as unlicensed users and can take advantage of times when the channels are not in use by PUs, without causing any impairment to the services required by the PUs.
The SUs may establish a communication link by rendezvous. Generally, rendezvous can be considered the process of two or more UEs completing a “handshake” to establish a communication link in an idle channel.
The implementation of a rendezvous between UEs is non-trivial and two systems are commonly applied. These are namely, the aided (or infrastructure-based) system and the unaided (or infrastructure-less) system. In an aided rendezvous system, the support for UEs to achieve a rendezvous is often performed by means of a central controller or a common central control channel. This simplifies the rendezvous process but is an approach vulnerable to jamming and overload attacks due to its low flexibility and scalability. On the other hand, an unaided system delegates to the UEs the task of finding available channels in a distributed manner. In practice, such an approach is often preferred because setting up a control channel in a DSA network might not be feasible.
The search of potential spectrum or channels for rendezvous without the aid of a control channel is commonly termed “blind rendezvous”. For instance, channel-hopping techniques offer a way to cope with blind rendezvous, ensuring the link establishment for a particular number of cases and limiting the maximum time-to-rendezvous (TTR). This is particularly true for symmetric hopping-based rendezvous techniques, in which the set of available channels of the hopping sequence is equal for all UEs.
For symmetric hopping-based rendezvous, another important aspect is time synchronization, which is achieved when all users in the system start their hopping sequence in a synchronous way, that is, without any delay among them. Synchronous rendezvous can be implemented by relying on a global synchronization signal, which may be provided over a control channel. However, asynchronous rendezvous is also possible, in which UEs attempt rendezvous without being synchronised with each other. Asynchronous rendezvous may be considered more general and does not demand additional infrastructure.
A generated orthogonal sequence (GOS) algorithm is disclosed in the asynchronous symmetric hopping-based rendezvous system of L. A. Dasilva and I. Guerreiro, “Sequence-based rendezvous for dynamic spectrum Access,” in 3rd IEEE Symposium on New Frontiers in Dynamic Spectrum Access Networks, 2008, pp. 1-7, October 2008. Therein, UEs, which may comprise a radio element, employ sequence-based generators to determine an order of channel search for rendezvous. The sequences are determined so as to ensure rendezvous. The GOS algorithm provides limited maximum TTR values as well as a bounded expected TTR. In N. C. Theis, R. W. Thomas, and L. A. DaSilva, “Rendezvous for cognitive radios,” in IEEE Transactions on Mobile Computing, vol. 10, no. 2, pp. 216-227, February 2011, a modular clock (MC) algorithm is disclosed, which generates channel hopping sequences using prime number modular arithmetic and random hopping rates. Therein, the MC algorithm is shown to outperform the GOS algorithm in terms of expected TTR with high probability. Nevertheless, a drawback of the MC algorithm is that it does not ensure rendezvous if by chance UEs set the same value to their random hopping rates.
A jump-stay (JS) algorithm is proposed in Z. Lin, H. Liu, X. Chu, and Y.-W. Leung, “Jump-stay based channel-hopping algorithm with guaranteed rendezvous for cognitive radio networks,” in IEEE International Conference on Computer Communications, 2011, pp. 2444-2452, April 2011. Such an algorithm assures rendezvous for any pair of users in an asynchronous symmetric channel hopping-based rendezvous system. Therein, channel hopping sequences comprise two parts: a jump pattern; and a stay pattern. The former is generated using prime number modular arithmetic and encompasses a subset of the channels available for hopping. The latter encompasses a single channel (determined by the user) which is consecutively visited a number of times. The hopping sequence is generated continuously by each user until rendezvous occurs. For the JS algorithm, the expected TTR presents smaller values than those obtained with the GOS and MC algorithms. However, the JS algorithm requires the run and build of the hopping sequence in a dynamic fashion while the GOS algorithm only requires that its generated sequence be known by all users. As such, the JS algorithm is complicated and computationally intensive.